FormoSat-3 / ROCSat-3 / COSMIC

The ROCSat-3/COSMIC (Republic of China Satellite-3 / Constellation Observing System for Meteorology, Ionosphere and Climate) is an international collaborative project between NSPO (National Space Program Office) of Taiwan and UCAR (University Corporation for Atmospheric Research) of the United States of America. Initiated in December 1997, the project will launch a LEO constellation of six microsatellites to collect atmospheric remote sensing data for operational weather prediction, climate, ionospheric (space weather monitoring), and geodesy research. NSPO is the prime sponsor and owner of the satellites. UCAR, located at NCAR in Boulder, CO, is primarily sponsored by NSF (National Science Foundation). Other partners in the project include JPL, NRL, USAF, NOAA, CWB (Central Weather Bureau of Taiwan), industry from both countries, universities, and other research organizations from the US, Taiwan, and other countries. ^{1) 2)}

The overall objective of ROCSat-3/COSMIC is to extend the low-cost research approach of refractive GPS radio occultation measurements (to derive important weather and climate research parameters, including atmospheric temperature, moisture, and pressure), that began with the GPS/MET instrument on Microlab-1 (launch April 3, 1995), to the next step by testing the ability of a constellation of six "ROCSat-3/COSMIC microsatellites with GPS/MET heritage" to provide the data needed to fully evaluate the impact of this promising new observational tool. A goal is also to demonstrate the utility of atmospheric/ionospheric limb soundings in operational weather prediction, space weather monitoring and space geodesy. In addition to carrying an advanced version of the JPL-developed GPS receiver for occultation measurement, each satellite will carry two tiny, simple secondary instruments (tri-band-beacon and photometer) which synergistically enhance the accuracy and utility of the ionospheric observations. A global data collection network and operations center will process space and ground observations and deliver products to users in real-time for operational impact studies. ^{3) 4) 5) 6) 7) 8) 9)}

Note: A public naming competition took place in Taiwan in 2004 with regard to the ROCSat satellite program. At the end of this contest, the ROCSat program was given the new name of FormoSat in December 2004. Hence, the ROCSat-3 constellation became FormoSat-3. In USA, FormoSat-3 is known under the name of COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate).

Figure 1: Artist's illustration of FormoSat-3/COSMIC spacecraft (image credit: NSPO)

Spacecraft:

Orbital Sciences Corporation (OSC) of Dulles, VA, USA is the prime contractor for the microsatellites, selected by NSPO. A joint team of the OSC and NSPO engineers designed and developed the satellites, with the early phase of the work performed at OSC and integration and test performed at NSPO. Several domestic industry companies of Taiwan were selected to participate in the project by providing satellite components. The following list gives an idea of the involvement: ^{10) 11)}

• Satellite computer (Acer Technologies Inc.)

• Mission interface unit (Acer Technologies Inc.)

• Solar sensor (Shihlin Electric & Engineering/eBright Corp.)

• Rechargeable storage battery (Shihlin Electric & Engineering/eBright Corp.)

• Current converter (Shihlin Electric & Engineering/eBright Corp.)

• Satellite antennas (Victory Industrial Corp.)

• Receiving coupler (Victory Industrial Corp.)

• Transmitting filter (Victory Industrial Corp.)

• Satellite heating elements (Yung Tien Industrial Co.)

The S/C structure is of Orbcomm heritage (MicroStar bus), a cylindrical shape of 1.03 m diameter and about 18 cm in height (width). On orbit, two solar panels deploy on each side of the satellite. All spacecraft are identical, with a mass of about 69 kg (including fuel). Each S/C features on-board propulsion to reach its final destination orbit. The ADCS (Attitude Determination and Control Subsystem) provides attitude knowledge (±5º roll and yaw, ±2º pitch) with an Earth limb sensor and a magnetometer. Power (46 W) is provided by a solar array and 10 Ah batteries. The propulsion system consists of two tanks to store propellant (hydrazine) and 4 small monopropellant thrusters. The design life is 2 years (5 years expendables). ^{12) 13)}

RF communications: The communication subsystem consists of an S-band receiver, an L-band transmitter, and a set of S-band and L-band antennas.

Figure 2: Inside view of the FormoSat-3/COSMIC spacecraft (image credit: NRL, UCAR) ¹⁴⁾

Launch: A launch of the constellation of 6 identical spacecraft took place on April 15, 2006 (UTC) from VAFB, CA. The low-cost nature of the mission required a special launch and deployment design of the constellation: ¹⁵⁾

• All six spacecraft were launched in a single shot by a Minotaur vehicle of OSC into one orbital plane. Minotaur has the capability to lift the six satellites into an initial circular parking orbit of about 500 km altitude with an inclination of 72º.

• This is being followed by a 13 month constellation deployment/distribution sequence. Each satellite will be separated from the launch vehicle individually. After in-orbit checkout, each satellite will be boosted by on-board thrusters to different altitudes ranging up to 800 km. The corresponding different rates of orbit nodal precession will then gradually drift the orbit planes apart, until a more-or-less even distribution of six orbit planes is achieved. The satellites will already start collecting atmospheric soundings during the orbit-adjustment and constellation distribution period. ¹⁶⁾

• The spacecraft deployment occurs in an inverse sequence (6th satellite first). The total deployment time is estimated to be 387 days. During the transition phase, there are 121 days for the 5th and 4th satellites in tandem flight, and 198 days for the 3rd and 2nd satellites in tandem flight. These temporary configurations are being used for the gravity study to determine the high order harmonics of the geopotential from the GPS data.

Figure 3: Spacecraft distribution in orbit after a series of initial orbit adjustments (image credit: NSPO)

Orbit of constellation: Circular orbits, altitudes of 800 km, inclinations of 72º; there are 6 operational planes with 1 satellite per plane, spaced 24º apart.

Ground segment:

The ground segment of FormoSat-3/COSMIC consists of three ground TT&C stations and a MOC (Mission Operations Center). The three stations are located in Taiwan, Fairbanks (Alaska), and Kiruna (Sweden). The MOC is embedded into NSPO's MMC (Multi-Mission Center). The MOC performs all S/C operations.

All science and some telemetry data is being sent to CDAAC (COSMIC Data Analysis and Archive Center) in Boulder, CO, and to TACC (Taiwan Analysis Center for COSMIC), a mirror site of CDAAC in Taiwan, located at CWB (Central Weather Bureau) in Taipei. The centers also receive data from a global network of ground GPS and TBB (Tri-Band Beacon Transmitter) receiving sites (the so-called fiducial network). The centers analyze the received data and distribute it to the principal investigators and to the science community for operational evaluation and research.

Table 1: FormoSat-3/COSMIC communication characteristics

During the initial two years of the mission, the science data will be made available through the FormoSat-3/COSMIC project websites without charge.

POD (Precise Orbit Determination) of the constellation is required for data analysis.

Mission status: The FormoSat-3 constellation is operating nominally as of 2007. All satellites are in good health and providing initial data. ^{17) 18)}

• As of Aug. 2007, the satellite constellation is approaching final deployment with only one more spacecraft, FM1, remaining its initial 500 km orbit (Figure 4). Presently the system is producing 1500-1700 good neutral atmospheric soundings per day with an average latency of about 2 hours. ¹⁹⁾

• Maneuvers continue to move the satellites into their final orbits. As of Jan. 2007, FM2 and FM5 are at 800 km altitude while FM6 is at 716 km. FM1, FM3 and FM4 are still at 518 km.

• The satellites are averaging about 1,200 soundings a day, in a nearly uniform global distribution, providing independent data over vast stretches of ocean and ice where there are no weather balloons. As the satellites approach their final positions, they will increase their output to about 2,500 soundings a day. ²⁰⁾

• The first set of occultation data from FormoSat-3 was obtained on April 21, 2006.

• Cosmic data became available to the public on July 28, 2006. JPL and its partners have begun processing Cosmic data into temperature and water vapor profiles of the atmosphere and measurements of the electron content of the ionosphere.

Figure 4: The deployment timeline of the FormoSat-3/COSMIC constellation (image credit: NSPO)

Sensor complement: (IGOR, TIP, CERTO/TBB)

The FormoSat-3/COSMIC constellation produces about 3000 soundings (minimum requirement of 2500/day) of bending angle and refractivity globally in all weather each day for at least one year after the spacecraft are placed in their final orbits. From these soundings, estimates of electron density in the ionosphere and temperature, water vapor and pressure in the stratosphere and troposphere will be derived. Desirable characteristics of these data include such items as: high accuracy, high vertical resolution, all weather (clouds and aerosols do not affect measurements), no calibration of instrument required, no instrument drift, require no first guess, modest cost.

Table 2: Science requirements of the FormoSat-3/COSMIC constellation

Table 3: Observational requirements of FormoSat-3/COSMIC

IGOR (Integrated GPS Occultation Receiver), based on the BlackJack GPS occultation receiver design of JPL and flown on such missions as CHAMP, SAC-C, and GRACE. IGOR, built by Broad Reach Engineering of Tempe, AZ, is the primary science instrument of the FormoSat-3/COSMIC constellation. The IGOR receivers, on FormoSat-3/COSMIC will be able to track all GPS satellites in view simultaneously, including two or more occulting satellites. It will operate fully autonomously, scheduling when to track which satellites and at what sampling rate based on its own known position and those of the GPS satellites. The instrument reports high-rate (50 Hz) dual frequency carrier phase measurements on the occulting links with sub-millimeter precision for accurate, high resolution profiling. Lower rate (0.1 Hz) phase measurements of all satellites in view are being collected for precise orbit determination (POD) at the 5-10 cm level.

IGOR tracks both GPS carrier frequencies (L1, L2) to separate the frequency-dependent (dispersive) ionospheric delay from the non-dispersive refractive delay of the neutral atmosphere. A patented "semi-codeless" technique is used to obtain precise measurements of the L2 signal, both carrier phase and pseudorange, with anti-spoofing turned on. In addition to these measurements, the GPS instrument can record GPS signal amplitudes for on-orbit ionospheric scintillation monitoring and correction of signal diffraction effects in post-processing. The instrument mass is 4.6 kg; size of about 20 cm x 24 cm x 10.5 cm; power of 16 W nominal, 23 W peak, antenna inputs: 4. ^{21) 22)}

Figure 5: Illustration of the IGOR instrument (image credit: Broad Reach Engineering)

The GPS radio occultation technique is based on the following principles: As a signal travels through the atmosphere it is retarded and bent. This results in a phase and Doppler shift, which can be measured very accurately by the GPS receiver aboard the LEO FormoSat-3/COSMIC satellites. Since the transmitter and receiver positions and velocities are accurately know from precise orbit determination, the signal bending angle alpha as a function of impact parameter, can be computed from the Doppler shift observed at LEO. From the basic bending angle versus impact parameter data, vertical profiles of refractivity as a function of tangent point radius can be derived. Further analysis converts refractivity to electron density in the ionosphere.

Figure 6: Occultation scheme of GPS signals with LEO satellites (image credit: Broad Reach Engineering)

TIP (Tiny Ionosphere Photometer) a nadir-viewing instrument, designed and built by NRL (Naval Research Laboratory), Washington, DC and Praxis Inc. TIP and TBB provide measurements of electron density, an important parameter of the upper atmosphere. The readings of TIP and TBB complement the primary IGOR instrument so that 3-D fields of electron density gradients between 90 and 750 km can be inferred.

TIP is a compact, narrow-band, ultraviolet photometer operating at the 135.6 nm wavelength (UV radiation). This emission is produced by recombination of O⁺ ions and electrons, which is the natural decay process for the ionosphere. At night, the strength of the emission is proportional to the product of the square of the peak electron density; during the daytime the emission is dominated by photoelectron impact excitation of atomic oxygen and is not useful for ionospheric sensing. ^{23) 24) 25) 26)}

Figure 7: TIP electronics module (top) and sensor module (image credit: NRL)

In particular TIP provides horizontal gradients in ionospheric electron density at the peak of the F2 layer, along the satellite orbit track. TIP measures the naturally occurring nighttime emission of neutral oxygen at 135.6 nm. This emission (airglow) is produced by the recombination of O + ions and electrons and is proportional to the square of the electron density in the ionospheric F region. Since horizontal gradients of electron density are a limiting error source for occultation inversions in the ionosphere, the combined analysis of TIP and GPS data promises improved retrievals of nighttime ionospheric profiles.

TIP is nadir-pointing with a 3.8º circular FOV providing a 30 km horizontal resolution from an orbital altitude of 800 km. The TIP sensor module consists of:

• Photomultiplier tube observing UV light

• Strontium fluoride filter passes 131-160 nm emissions

• Very high sensitivity ~150 counts/s/Rayleigh

Figure 8: Illustration of the TIP filter wheel (image credit: NRL)

CERTO/TBB (Coherent Electromagnetic Radio Tomography/Triband Beacon Transmitter), designed and built at NRL. CERTO/TBB transmits phase data measurements at 150, 400 and 1067 MHz (VHF, UHF, L-band) which can be received at ground stations worldwide. These data are converted to line-of-sight TEC (Total Electron Content) observations that can be processed with 2-dimensional ionospheric tomography techniques. CERTO/TBB data can also be combined with the other ionospheric observations in tomographic and physical data assimilation models to compute global four-dimensional electron density fields. ^{27) 28) 29) 30) 31)}

The FormoSat-3/COSMIC instrument suite permits three-dimensional tomography of the ionosphere with unprecedented resolution and accuracy. FormoSat-3/COSMIC data will be highly complementary to other satellite sounding systems, including radiometric sounders on the POES and GOES series satellites of NOAA. The independence and the high-vertical resolution of the radio occultation soundings complement the high horizontal resolution of the radiometric soundings and together the two systems can likely be combined to yield composite soundings of temperature and water vapor with unprecedented accuracy, horizontal and vertical resolution, and global coverage.

Figure 9: Photo of the CERTO instrument (image credit: NRL)

Figure 10: CERTO/TBB accommodation on FormoSat-3/COSMIC (image credit: NRL)

Figure 11: Joint CERTO/TBB, GPS-GOX, TIP operations on FormoSat-3/COSMIC (image credit: NRL)

2) C. Rocken, Y. H. Kuo, W. S. Schreiner, D. Hunt, S. Sokolovskiy, C. McCormick, "COSMIC System Description," Special issue of TAO (Terrestrial, Atmospheric and Oceanic Science), Vol. 11, No. 1, March 2000, pp.21-52

3) G. A. Hajj, L. C. Lee, X. Pi, L. J. Romans, et al., COSMIC GPS Ionospheric Sensing and Space Weather," Special issue of TAO (Terrestrial, Atmospheric and Oceanic Science), Vol. 11, No. 1, March 2000, pp.235-272

4) Y. K. Kuo, L. C. Lee, "A Constellation of Microsatellites Promises to Help in a Range of Geoscience Research," EOS Transcriptions, AGU, Vol. 80, No. 40, Oct. 5, 1999, pp. 467-471

5) http://www.cosmic.ucar.edu/

6) Information provided by Paul Chen of NSPO

7) E. B. Pavlis, C. Chao, C. Hwang, C. Liu, C. Shum, C. Tseng, M. Yang, "Geodetic applications of the ROCSat-3/COSMIC mission, Towards an Integrated Global Geodetic Observing System (IGGOS)," International Association of Geodesy Symposia, Vol. 120, editors: R. Rummel, H. Drewes, W. Bosch, H. Hornik, pp. 214-217, Springer-Verlag Berlin, Germany, October, 1998

8) http://www.nspo.org.tw/2005e/projects/project3/intro.htm

9) Y. H. Kuo, C. Rocken, R. A. Anthes, "Constellation Observing System for Meteorology, Ionosphere and Climate (COMIC)," URL: http://ams.confex.com/ams/pdfpapers/82660.pdf

10) http://www.nspo.org.tw/2005e/projects/project3/component.htm

11) http://www.ucar.edu/news/releases/2006/cosmicfacts.shtml

12) http://www.orbital.com/SatellitesSpace/LEO/FORMOSAT-3/index.html

13) FormoSat-3/COSMIC, http://www.orbital.com/NewsInfo/Publications/FORMOSAT-3_Fact.pdf

14) P. A. Bernhardt, C. L. Siefring. A. Yau,"Space Based Systems for Ionospheric Density and Scintillation Mapping in Conjunction with Incoherent Scatter Radars," AMISR Science Planning Meeting, Asilomar, CA, Oct. 12, 2006 URL: http://www.amisr.com/meetings/2006asilomar/presentations/Bernhardt/AMISRSpaceBasedPoster.ppt

15) A. M. Wu, C. J. Shieh, V. Chu, "ROCSat-3 Constellation Design and Data Simulation," Proceedings of 53rd IAC and World Space Congress, 2002, Oct. 10-19, 2002, Houston, TX, IAF-02-A.7.06

16) Note: Earth oblateness is the reason for the orbital plane drifts. The nodal precession, a well-known gravity phenomenon, is for instance being used by all spacecraft in sun-synchronous orbit to compensate for the Earth's revolution around the sun (about 0.9856^º per day).

17) C.-H. Vicky Chu, S.-K. Yang, C.-J. Fong, N. Yen, T.-Y. Liu, W.-J. Chen, D. Hawes, Y.-A. Liou, B. Kuo, "The Most Accurate and Stable Space-Borne Thermometers - FORMOSAT-3/COSMIC Constellation," Proceedings of the 21st Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 13-16, 2007, SSC07-VII-1

18) Annual AMS (American Meteorological Society) Meeting 2007, San Antonio, TX, USA, Jan. 15-18, 2007, N. Yen, URL: http://www.cosmic.ucar.edu/AMS2007/AMS_NICK_011507.ppt

19) Latest Status update - Aug. 23, 2007, URL: http://www.cosmic.ucar.edu/

20) An-Ming Wu, Lance Wu, "Integrated Mission Planning for FormoSat-2 Imaging Satellite and FormoSat-3 Meteorological Constellation," Proceedings of the 57th IAC/IAF/IAA (International Astronautical Congress), Valencia, Spain, Oct. 2-6, 2006, IAC-06-B1.6.09

21) http://www.broad-reach.net/igor06.html

22) http://www.cosmic.ucar.edu/systems/IGOR_Flyer.pdf

23) C. Coker, K. F. Dymond, S. A. Budzien, D. Chua, "First Observations of the Ionosphere Using the Tiny Ionospheric Photometer," Taipei, Taiwan, FormoSat-3/COSMIC Workshop 2006 - Early Results and IOP Campaigns, Nov. 28-Dec. 1, 2006, URL: http://www.cosmic.ucar.edu/oct2006workshop/presentations/Coker_Clayton_20061017.ppt

24) P. C. Kalmanson, S. A. Budzien, C. Coker, K. F. Dymond, "The tiny ionospheric photometer instrument design and operation," Proceedings of SPIE, `Instruments, Science, and Methods for Geospace and Planetary Remote Sensing,' Carl A. Nardell, Paul G. Lucey, Jeng-Hwa Yee, James B. Garvin, editors, Vol. 5660, Bellingham, WA, Dec. 2004, pp. 259-270

25) C. Coker, K. F. Dymond, S. A. Budzien, "Using the Tiny Ionospheric Photometer (TIP) on the COSMIC Satellites to Characterize the Ionosphere," American Geophysical Union (AGU) Fall Meeting San Francisco, CA, Dec. 6-10, 2002

26) K. F. Dymond, J. B. Nee, R. J. Thomas, 2000: "The Tiny Ionospheric Photometer: An Instrument for Measuring Ionospheric Gradients for the COSMIC Constellation," Terrestrial, Atmospheric and Oceanic Sciences, Vol. 11, 2000, pp. 273-290.

27) P. A. Bernhardt, C. E. Coker, "New TEC Data Sources from Radio Beacon Monitors of the Ionosphere," LWS Geostorm CDAW and Conference Florida Tech, Melbourne, FL, March 8, 2007, URL: http://www.cosmic.ucar.edu/aug2002workshop/presentations/rocken_pres.ppt

28) P. A. Bernhardt, C. A. Selcher, S. Basu, G. Bust, S. C. Reising, 2000: "Atmospheric studies with the Tri-Band Beacon instrument on the COSMIC constellation," Terrestrial, Atmospheric and Oceanic Sciences, Vol. 11, No 1, March 2000, pp. 291-312

29) P. A. Bernhardt, C. L. Siefring, "The CERTO and CITRIS Instruments for Radio Scintillation and Electron Density Tomography from the C/NOFS, COSMIC, NPSAT1 and STPSAT1 Satellites," The 2004 Joint Assembly (of CGU, AGU, SEG and EEGS), Montreal, Canada, May 17.21, 2004

30) P. A. Bernhardt, C. L. Siefring, T. W. Garner, T. L. Gaussiran, J. Secan, F. Smith, K. Groves, "First Results for the TBB/CERTO Beacon Experiment on FormoSat- 3/COSMIC," AGU (American Geophysical Union) Fall Meeting, 2006, San, Francisco, CA, USA, Dec. 11-15, 2006

31) P. A. Bernhardt, C. L. Siefring, J. D. Huba, C. A. Selcher, CITRIS: The Cosmic Companion for LEO Radio Occultation," COSMIC Radio Occultation Workshop, Aug. 21, 2002, Boulder, CO, USA, URL: http://www.cosmic.ucar.edu/aug2002workshop/presentations/